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TEK 420A Trigger problems

Posted: October 23rd, 2017, 12:48 pm
by willhelmx
Hey Guy´s

Taking a chance to write here about a problem i have with one 420A Tektronics Scope

We have used this for a long time and it has always worked fine, but the latest months it´s been starting to behave strange, when you try to trig any signal outside the "autoset" funktion wich gives you like 2,5G slow cycle, the scope says Trig´d but the scree rolls from right to left or viceversa, we went ahead and changed all caps in the sweet thing, wich needed to be done anyway = original scope, but it dident do any change..

We cannot find any real electrical problem with it, nor any hints in the service manual, now on the old scopes this could be adjusted inside, on this one the service manual mention adjustment software via the Gpbib port, and i can imagine that this could be a adjustment kindda thing, but if so ... can one today set up a gpib connection on a win 10 system and talk to this thing ?????

Any idaea would be helpful

Best regards

/ Anders

Re: TEK 420A Trigger problems

Posted: April 1st, 2018, 1:00 am
by tektronixjim
Hi Wilhelm,
Trigering problem can be caused bij bad capacitors in the Power Unit. A PSU problem affects everything in a device. Some parts do not matter, others go strange. In self-testing the scope tests where the design is critical. Whether the test is simply wrong or another error affects the measurement so that another test fails. I have had a PAT tester in which they do a relay self-test. To my surprise, the device did not see that the relays were removed. My own Marconi SA gave all kinds of errors caused by the PSU (about 150 bad elcos replaced). Afterwards he occasionally gave an error which according to the service manual was "somewhere" in the digital part. I could not find anything. It turned out that there were two bad transistors in the analog log amp. You would expect that it always goes wrong but apparently the system could correct a lot.
Regards
Harry